Amazon's Kindle is a wireless reading device that does allow the user to adjust font size, so it might be appropriate for your son. The Kindle offers variable font size, with the largest font appearing to be about the size of a typical large print book.
The Kindle is also rather expensive, so you may want to do a little research first. If your son needs something larger than a typical large print book, the Kindle's largest font may not be what he needs. Another good place to do some research and ask questions is the Kindle discussion board on Amazon's website. Here you can ask other users about their experiences, talk to other parents who may use the Kindle with their child, or even arrange to see a Kindle in your city so you can try it before you buy. Find other reading hardware and software options in the article, Reading Software: Finding the Right Program.
If your child has a diagnosed print disability, he is eligible to receive texts in alternate formats through his special education program. Discuss this option with the school. Learn more in these articles for parents: Accessible Textbooks: A Guide for Parents of Children with Learning Disabilities and Making the Written Word Easier for Readers with Print Disabilities.